Synthesis, complex formation kinetics and thermodynamic study of some acyclic polyamine and N2O2 ligands with copper(II)

Article abstract of DOI:10.1080/00958972.2010.535144

The polydentate ligands, 3-(2-aminocyclohexylamino)-2-(2-aminocyclohexyl aminomethyl) propionic acid (L¹), 4,7,10-triazatridecanedinitrile trihydrochloride (L²), and 2,2'-(ethane-1,2-diyl) bis(methylazanediyl) diethanol (L³) were prepared and their structures investigated by FT-IR, NMR, and MS. The kinetics of complex formation between Cu(II) and L¹, L², and L³ were investigated in acidic aqueous solutions using the stopped-flow method. The stability constants of the complexes were determined by spectrophotometric titration (T = 293 K, = 0.1 mol L^-1 NaClO₄), using a diode array UV-Vis spectrophotometer equipped with peristaltic pump and pH meter. The stability constants for the complexes were CuL¹ CuL² CuL³. Activation enthalpies (H#) of these complexes were 55 kJ mol^-1 for CuL¹, 61 kJ mol^-1 for CuL ², and 36 kJ mol^-1 for CuL³, respectively.

Full text of DOI:10.1080/00958972.2010.535144

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Synthesis, complex formation kinetics
and thermodynamic study of some
acyclic polyamine and N2O2 ligands
with copper(II)
Hava Ozay a & Yakup Baran a
a Department of Chemistry, Art and Science Faculty , Onsekiz Mart
University , 17100 Canakkale, Turkey
Published online: 19 Nov 2010.
To cite this article: Hava Ozay & Yakup Baran (2010) Synthesis, complex formation kinetics and
thermodynamic study of some acyclic polyamine and N2O2 ligands with copper(II), Journal of
Coordination Chemistry, 63:24, 4299-4308, DOI: 10.1080/00958972.2010.535144
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Journal of Coordination Chemistry
Vol. 63, No. 24, 20 December 2010, 4299–4308
Synthesis, complex formation kinetics and thermodynamic study
of some acyclic polyamine and N2O2 ligands with copper(II)
HAVA OZAY and YAKUP BARAN*
Department of Chemistry, Art and Science Faculty, Onsekiz Mart
University, 17100 Canakkale, Turkey
(Received 23 July 2010; in final form 12 October 2010)
The polydentate ligands, 3-(2-aminocyclohexylamino)-2-(2-aminocyclohexyl aminomethyl)
propionic acid (L1), 4,7,10-triazatridecanedinitrile trihydrochloride (L2), and 2,20-(ethane-1,2-
diyl) bis(methylazanediyl) diethanol (L3) were prepared and their structures investigated by
FT-IR, NMR, and MS. The kinetics of complex formation between Cu(II) and L1, L2, and
L3 were investigated in acidic aqueous solutions using the stopped-flow method. The stability
constants of the complexes were determined by spectrophotometric titration (T ¼ 293 K,
ꢀ ¼ 0.1 mol Lꢀ1 NaClO4), using a diode array UV-Vis spectrophotometer equipped with
peristaltic pump and pH meter. The stability constants for the complexes were
CuL1 4 CuL2 4 CuL3. Activation enthalpies (DH#) of these complexes were 55 kJ molꢀ1 for
CuL1, 61 kJ molꢀ1 for CuL2, and 36 kJ molꢀ1 for CuL3, respectively.
Keywords: Complexation; Polyamine; Kinetics; Stability constants; Stopped-flow;
Spectrophotometric titration
1. Introduction
Polyamines form an important class of compounds due to the roles played by them as
polyprotic bases [1], biologically important compounds [2, 3], and sensors for the
detection of metal ions and metal ion complexation [4–8]. Metal complexes of
polyamine ligands have applications in many areas from coordination chemistry to
bioinorganic chemistry. Transition metal complexes of multidentate ligands are used as
model systems for many metalloenzymes [9–11], in luminescence sensing, in light-
emitting devices, in inter-metallic communication, as a catalyst, in molecular electronics
[12, 13], and in coordination polymer chemistry [14].
Copper complexes play an important role in biological systems [15]. Catechol oxidase
found in plants is a binuclear copper complex. This enzyme catalyzes the oxidation of
catechols with molecular oxygen [16]. There is a deficiency in understanding the
*Corresponding author. Email: yakupbaran@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.535144

4300
H. Ozay and Y. Baran
structures, oxygen activity, and functions in biological systems of many enzymes which
contain different numbers of metal centers. Preparation of a synthetic model compound
of an enzyme is one method to overcome this deficiency. Preparation of model
compounds is important [17, 18] for a better understanding of the behavior of
these enzymes.
Here, we report the synthesis of L1, L2, and L3 (figure 1) and the kinetics of the
complex formation and stability of their copper(II) complexes. Kinetic inertness and
thermodynamic stability are important properties of complexes with potential use in
biological systems, as catalysts in organic reactions, or as sensors for metal ion
recognition and magnetic resonance imaging (as a contrast agent) [15, 19–22]. Although
there is a great deal of information available about the stability, kinetics of formation,
decomposition, and hydrolysis of mononuclear complexes formed with polyaza ligands,
we tried to add some contribution to this field. Therefore, Cu(II) complexes of L1, L2,
and L3 were studied, as they can be considered model compounds. In this article, the
kinetic and thermodynamic properties of these complexes are reported.
2. Experimental
2.1. Chemicals and methods
All reagents were obtained commercially and used as received. Solvents were purified
according to the standard methods prior to use. Mass spectra were measured with a VG
Autospec-oa-TOF Mass Spectrometer (ESI). Nuclear magnetic resonance (NMR)
spectra were measured with a Varian 300 MHz spectrometer. Elemental analyses were
performed on a LECO, CHNO organic element analyzer. Reaction kinetics and
spectrophotometric titration were measured with UV-Vis, HP 8453 Diode Array
Spectrophotometer, and the Pro-K.2000 Stopped-Flow accessory with pneumatic drive
system (Applied Photophysics) for rapid kinetics studies. For the spectrophotometric
titration, acid or base solution was added to a 1-cm quartz cell with peristaltic pump
(Cole Palmer, Masterflex) and the pH values of the solutions were measured with an
Orion pH meter combined with a Metrohm semi-micro electrode. FT-IR spectra were
measured with a Perkin Elmer BXII spectrometer.
COOH
H N
2
HN
HN
N
H3C
CH3
H
N
N
HN
NC
NH
CN
H N
2
H
OH
HO
1
2
3
L
L
L
Figure 1. Structures of ligands.

Synthesis, complex formation kinetics and thermodynamic study
2.2. Synthesis of ligands
2.2.1. Synthesis of 3-(2-aminocyclohexylamino)-2-(2-aminocyclohexylaminomethyl)
4301
1
propionic acid, L1 4HCl 2H2O. L was prepared by template synthesis from
bis(cyclohexane-1,2-diamine)copper(II), triethylamine, diethylmalonate, and formalde-
hyde in methanol. The synthetic pathway followed for L1 is shown in figure 2. An
aqueous solution of copper(II)perchlorate hexahydrate (11.28 g, 30.00 mmol) was added
to a solution of 1,2-diaminocyclohexane (6.85 g, 60.00 mol) in deionized water (300 mL)
on a magnetic stirrer. The reaction mixture was warmed to 50ꢁC, stirred at this
temperature for 2 h, and cooled to room temperature and bis(cyclohexane-1,2-diamine)
copper(II)perchlorate (1) was separated by filtering.
.
.
Solution of bis(cyclohexane-1,2-diamine)copper(II)perchlorate (6.00 g, 12.00 mmol)
in 250 mL methanol was heated to 50ꢁC while stirring magnetically and to this solution,
triethylamine (6 mL, 43.05 mmol) and diethylmalonate (1.90 mL, 12.00 mmol) were
added. Then, a solution of formaldehyde (37% aqueous solution, 3 mL) in methanol
(50 mL) was added dropwise to the reaction mixture and the reaction mixture was
stirred for 16 h at 50ꢁC. The color of the reaction mixture converted to purple-red
during this time. After evaporation of the solvents, the purple-colored crude product
was obtained. Five grams of crude product were dissolved in deionized water (250 mL)
and the solution was diluted to 2 L with deionized water. Then the solution was passed
through a column (35 ꢂ 3.5 cm) of SP Sephadex C-25 resin (Naþ form) and the column
was eluted with 0.2 mol Lꢀ1 NaClO4 solution. After a while, two bands, one narrow and
one broad, were observed. Both bands were collected. A small amount of macrocyclic
compound also formed. The solvent of the acyclic compound was evaporated and
the compound was dried. Five milliliters of triethylamine were diluted to 25 mL
with deionized water and added to the solution of acyclic compound (3.5 g) in
H2N
H2N
NH2
NH2
COOC2H5
COOC2H5
ii
i
H2N
H2N
NH
NH
(ClO4)2
+
Cu(ClO4)2.6H2O
Cu
(ClO4)2
Cu
H2N
NH2
1
2
iii
COOH
COOH
H
H2N
H2N
NH
NH
H2N
H2N
HN
HN
iv
(ClO4)2
Cu
COOC2H5
L1
Figure 2. Preparation of L1. Reagents and conditions: (i) H2O, 50ꢁC, 2 h; (ii) N(C2H5)3, CH2(COOC2H5)2,
CH2O, CH3OH, 50ꢁC, 14 h; (iii) N(C2H5)3, CH3OH, 60ꢁC, 12 h; and (iv) Zn, HCl.

4302
H. Ozay and Y. Baran
methanol (200 mL). The reaction mixture was stirred on a magnetic stirrer at 60ꢁC for
12 h. At the end of this time, the mixture was cooled to room temperature and was
diluted to 2 L with deionized water. The diluted solution was passed through a column
(35 ꢂ 4 cm) of SP Sephadex C-25 resin (Naþ form) and the column was eluted with
0.2 mol Lꢀ1 NaClO4 solution. The band observed in the column was collected and the
solution was concentrated to 300 mL by rotary evaporation. This solution and
3 mol Lꢀ1 HCl solution were simultaneously added dropwise from dropping funnels
over 2 h to Zn powder while stirring on a magnetic stirrer at room temperature. Then,
the solution was heated to 50ꢁC and stirred for 30 min. The solution was cooled to room
temperature and was filtered on Celite to remove Cu and residual Zn. The clear solution
was diluted to 2 L with deionized water and the solution was passed through a column
(35 ꢂ 3 cm) of Dowex 50 W ꢂ 2 resin (Hþ form); the column was eluted for a while with
deionized water and afterward with 1 mol Lꢀ1 HCl solution to remove Znþ2. Elution
continued until no further Znþ2 was present (checked by the addition of NaOH solution
to the eluant in order to observe Zn(OH)2). After the formation of gelatinous Zn(OH)2,
the column was eluted with 3 mol Lꢀ1 HCl. Evaporation of the solvent by rotary
evaporator gave a white crude product and this product was twice recrystallized with
hotmethanolandL1wasobtainedasawhitepowder(C16H32N4O2 ꢃ 4HCl ꢃ H2O),L1.Yield:
2.1 g,52%.C16H35Cl4N4O2 ꢃ 4HCl ꢃ 2H2O(CalcdC,38.95;H,7.97;andN,11.36)found%:
C, 38.79; H, 7.81; and N, 11.44). 13C-NMR (D2O) 19.26(2C); 19.37(2C); 22.87; 23.09;
26.05(2C);33.67;40.63;41.57;47.44(2C);54.61;56.38;and173.34;m/z:314(Mþ),FTIR(ATR,
cmꢀ1):ꢁ(COOH):2911vs(br),2017 m,1612s,and1514 s.
2.2.2. Synthesis of 4,7,10-triazatridecanedinitrile trihydrochloride, L2 3HCl. 4,7,10-
.
Triazatridecanedinitrile trihydrochloride was synthesized by the condensation of
diethylenetriamine (dien) and acrylonitrile according to the literature [23].
Acrylonitrile (3.19 g, 60.00 mmol) was added dropwise to the dien (2.58 g, 25 mmol)
while stirring magnetically, and the mixture was stirred for 20 h at room temperature.
The crude product was purified as trihydrochloride by recrystallization from methanol/
water/HCl and L2 ꢃ 3HCl was obtained as a white powder. Yield: 3.20 g, 40%.
C10H19N5 ꢃ 3HCl (Calcd C, 37.69; H, 6.96; and N, 21.98) found %: C, 37.43; H, 7.11;
and N, 21.84). 13C-NMR (D2O) 120.25(2C); 46.87(2C); 46.51(2C); 46.28(2C); and
17.96(2C); m/z: 210 (Mþ), FTIR (ATR, cmꢀ1): ꢁ(NHþ3 ): 2667, 2435; ꢁ(CN): 2661.
2.2.3. Synthesis of 2,20-(ethane-1,2-diyl) bis(methylazanediyl))diethanol, L3. Ethylene
oxide (3.02 g, 60.00 mmol) was added dropwise to a solution of N,N0-dimethylethyle-
nediamine (2.00 g, 22.68 mmol) in methanol (50 mL) at 0ꢁC, stirred at 0ꢁC for 12 h, and
warmed to room temperature; the solvent volume was reduced by a rotary evaporator.
The solution was left for crystallization and L3 was obtained as a viscous oil. Yield:
2.7 g, 76%. C8H20N2O2 (Calcd C, 38.88; H, 8.16; N, and 11.33) found %: C, 38.79;
H, 7.88; N, and 11.28). 13C-NMR (D2O) 58.58(2C); 58.09(2C); 53.63(2C); and
41.72(2C); m/z: 176.99 (Mþ), FTIR (ATR, cmꢀ1): ꢁ(OH): 3270.
2.2.4. Synthesis of copper(II)- L1 complex. L1 ꢃ 4HCl (0.82 g, 1.35 mmol) and
Cu(ClO4)2 ꢃ 6H2O (0.5 g, 1.35 mmol) were dissolved in 50 mL water. The pH of the
solution was adjusted to 8 with aqueous NaOH (2.5 mol Lꢀ1) and the solution was

Synthesis, complex formation kinetics and thermodynamic study
4303
allowed to stir at room temperature overnight. The solution was then diluted to 2 L with
water and loaded onto a column of SP-Sephadex (C-25) cation exchange resin
(4 ꢂ 30 cm). The column was then washed with 1 L of water. Elution with 0.32 mol Lꢀ1
NaClO4 resulted in two bands. Both bands proved to be stable in acidic solution and
were thus identified as being acyclic products. Both solutions were reduced in volume to
ꢄ50 mL and then left to stand. Within 24 h, the first solution precipitated a purple solid,
which was shown (IR spectrum: 1641 cmꢀ1, COOꢀ) to be the deprotonated form. A few
days later, the second solution precipitated a pink solid which was shown (IR spectrum:
1711 cmꢀ1, COOH) to be the protonated form of the complex. Visible spectra,
measured at pH ꢄ10 and then pH ꢄ3, showed identical ꢂmax, confirming that both
products were of the same ligand in different protonation states. The protonated
complex was characterized by microanalysis (Found %C, 31.18; %H, 5.4; and %N, 9.5.
C16H28N4O8Cl2Cu ꢃ 2H2O requires %C, 31.66; %H, 5.31; and %N, 9.23). Electronic
spectrum ꢂmax (H2O, pH 7) 260, 530 nm.
2.3. Kinetics
Complex formation kinetics of Cu(II) complexes with L1, L2, and L3 were followed
under second-order conditions (1.2 mmol ligand and metal concentrations) at four
different temperatures with the combination of an Agilent HP 8453 Diode Array
UV-Vis spectrophotometer and Pro-K.2000 rapid kinetics system with pneumatic drive
system. The kinetics mode of the HP was used with activated external trigger. Global
kinetic analysis and simulation software were used for kinetic data analysis. A 1-cm
quartz cell was used in the kinetic measurements, thermostated ꢅ0.1ꢁC using an
external circulating water bath. Measurements were made at multiple wavelengths and
kinetics traces were acquired at all wavelengths. Argon-saturated deionized water was
used in the kinetic study. Each kinetic run was repeated at least three times. Ionic
strength of the solutions was adjusted with 0.1 mol Lꢀ1 NaClO4. All the kinetic
measurements were followed by a UV-Vis spectrophotometer with stopped-flow
accessory in the ꢂmax of the absorption peak (table 1). Figure 3 shows the typical 3-D
spectral changes and 3-D residue at a pH of 4.00 and 0.1 mol Lꢀ1 ionic strength for the
formation of CuL1.
2.4. Spectrophotometric titrations
Stability constants of the complexes were measured with an automatic titration setup,
consisting of a computer interfaced to the Agilent HP 8453 Diode Array
Table 1. Maximum absorbance peaks for the complex forma-
tion reactions of CuL1, L2, and L3 studied at 25ꢁC and
I ¼ 0.1 mol Lꢀ1 NaClO4 in water.
Complex
pH
UV-Vis (ꢂmax, nm)
L1
L2
L3
4.00
3.22
3.63
260
265
263

4304
H. Ozay and Y. Baran
Figure 3. (a) 3-D spectral changes during complex formation of CuL1 pH ¼ 4.00, I ¼ 0.1 mol Lꢀ1 NaClO4
and (b) 3-D display of residuals to kinetic fit of CuL1 formation reaction, [M] ¼ [L] ¼ 1.20 m mol Lꢀ1
.
Spectrophotometer with a stirred thermostated cell holder, a peristaltic pump (Cole
Palmer, Masterflex), and an Orion pH meter combined with a Metrohm semi-micro
electrode. The electrode was calibrated with a pH of 4.0 and 7.0 buffers for
measurements in aqueous solutions. An argon-saturated acidic mixture of the ligand
(1.2 mmol) and the metal ion (1.2 mmol) containing 0.1 mol Lꢀ1 NaClO4 for adjustment
of ionic strength was titrated with a base (0.1 mol Lꢀ1 NaOH) in a 1-cm quartz cell. The
cell compartment was thermostated to 20 ꢅ 0.1ꢁC during titration. The cell was closed
with a Teflon cap containing a pH electrode and a capillary tip from the peristaltic
pump. The UV-Vis spectrum was determined during the titration at 60 s intervals over
the wavelength range 350–1100 nm. Figure 4 shows typical absorption spectra of CuL1

Synthesis, complex formation kinetics and thermodynamic study
4305
1.50
1.30
1.10
0.90
0.70
0.50
0.30
0.10
10.0
8.0
6.0
pH
4.0
2.0
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 4. 3-D absorption spectra of CuL1 (ꢂ ¼ 675 nm) during spectrophotometric titration as a function of
pH at 293 K, I ¼ 0.1mol Lꢀ1 NaClO4.
during spectrophotometric titration as a function of pH at 0.1 mol Lꢀ1 ionic strength
at 20ꢁC. The measurements were made from pH 2.0 to 11.0. Triplicate data analyses
were performed for each complex. Data analysis was carried out using the nonlinear
least-square fitting program Specfit/32. An initial guess for the equilibrium constant
was entered and iteratively refined until the best fit indicated by least squares was
achieved.
3. Results and discussion
3.1. Kinetics
Kinetic experiments were carried out under second-order conditions, in the pH range
3–4. In the investigated range of pH, many species with different protonation numbers
may occur, but these species have low concentrations which may not affect the reaction
rate. The kinetic behavior was modeled by the Eigen–Wilkins mechanism. The solvated
metal ion reacts with the ligand rapidly forming an outer sphere complex. The ligand
then enters the inner sphere where the ligand replaces a coordinated solvent molecule.
The rate of this interchange controls the rate of complex formation. For the polydentate
ligand, there is another step during which ring closure occurs and determines the rate of
reaction. Kinetic experiments show that formation of CuL complexes occurs in a single
kinetic step and values derived from second-order rate constants includes all species in
solutions. The values are 392.1 ꢅ 0.3 for CuL1, 82.7 ꢅ 0.3 for CuL2, and 818.7 ꢅ 0.3 for

4306
H. Ozay and Y. Baran
Table 2. Rate constants and activation parameters for the formation of Cu(II)
complexes at 25ꢁC and I ¼ 0.1 mol Lꢀ1 NaClO4 in water.
Complex
k ((mol Lꢀ1 ꢀ1
)
sꢀ1
)
DH# (kJ molꢀ1
)
DS# (J molꢀ1 K)
L1
L2
L3
392.1 ꢅ 0.3
82.7 ꢅ 0.2
818.7 ꢅ 0.2
55
61
36
34
39
67
L3 (table 2). Enthalpies of activation of these complexes are 55, 61, and 36 kJ molꢀ1
,
respectively. The positive entropy of activation indicates that the reaction proceeds in a
dissociative pathway.
3.2. Stability of the complexes
Accurate determination of the stability constants is required in various chemical and
biochemical fields. Spectrophotometric titrations have often been employed and they
consist of recording and evaluating spectra derived from the titration system as
a function of variables, such as pH. The range of techniques available through
spectrometry has been covered extensively [24]. Spectrophotometric methods are, in
general, highly sensitive and suitable for studying protonation equilibria in
solutions [25].
The L1 and L2 polyamines were prepared as described in the experimental section.
L3 was prepared using a procedure similar to that previously described for the related
diamine compounds. The stability constant values of the Cu(II) complexes were
obtained from analysis of spectrophotometric titrations (table 3). These values were
derived from the analysis of a solution containing 1 : 1 molar ratios at 20ꢁC in the
presence of 0.1 mol Lꢀ1 NaClO4. The equilibrium model for Cu(II) complexes of these
ligands is simple and includes only formation of CuL and HCuL. The stability of CuL1
is about two orders of magnitude higher than CuL2 and six orders higher than CuL3.
These observations can be explained from the higher basicity of L1. The same trend can
be observed for the HCuL species. For the open-chain polyamine complexes, the
protonation pattern is such that electrostatic repulsion is minimized. Protons attach
first to the terminal primary amine, and then to the secondary amines. L1 is potentially
a pentadentate ligand with carboxylate pendant. N-alkylation, changing the basicity of
the nitrogen donor, could change the stability of CuL3. The methyl groups, apart from
causing a decrease in basicity of the secondary amine donor, may introduce steric
crowding which can cause a low stability constant.
4. Conclusion
As a whole, the stability constants and kinetic data used in this article indicate that
changes in the donor for the open-chain tetradentate ligand lead to a change in the
properties of their Cu(II) complexes. For L1, the single metal ion is tightly coordinated
to four amines to yield a very stable complex. N-alkylation and two oxygen donors in

Synthesis, complex formation kinetics and thermodynamic study
4307
Table 3. Stability constants of the complexes at 20ꢁC, I ¼ 0.1 mol Lꢀ1 NaClO4.
Ligand
L1
Metal
Cu2þ
Cu2þ
Cu2þ
M, L, H
Log ꢃMLH
Equilibrium, K
Log KMLH
ꢀ
ꢀ!
110
111
110
111
110
111
17.11
23.42
15.43
19.51
11.44
15.31
M þ L
ML
17.1 ꢅ 0.1
6.3 ꢅ 0.1
15.4 ꢅ 0.2
4.1 ꢅ 0.1
11.4 ꢅ 0.3
3.9 ꢅ 0.1
ꢀ
ML þ H
MLH
MLH
MLH
ꢀ!
L2
M þ L
ML
ꢀ
ꢀ!
ꢀ
ML þ H
ꢀ!
L3
M þ L
ML
ꢀ
ꢀ!
ꢀ
ML þ H
ꢀ!
L3 decrease the stability of the mononuclear complex by several orders of magnitude.
By contrast, L3 reacts faster with Cu(II) than L1. These results demonstrate that the
kinetics and stability of the complexes can be finetuned by introducing minor
modifications in the ligand structures.
Acknowledgments
The authors thank the Scientific and Technological Research Council of Turkey
(TUBITAK) for providing financial support (project no: 104T389).
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